1. Field of the Invention
This invention relates generally to an apparatus for use in imaging surface pattern impression and more specifically to such an apparatus which is highly suited for fingerprint imaging.
2. Description of the Prior Art
Various techniques for identifying individuals have been proposed and, among other things, fingerprint identification has proven one of the most reliable methods. An apparatus for identifying individuals through the use of fingerprint images, has been proposed wherein a fingerprint image is developed in the apparatus and automatically compared, using pattern recognition technique, with previously stored fingerprint images. Such an apparatus is provided with a fingerprint image acquisition terminal wherein a finger is placed in contact with a surface of an optical element internally illuminated (hereinafter, the surface of the optical element will sometimes be referred to as a window). The fingerprint image is obtained by utilizing the change in boundary conditions which occur on the surface or window on which a finger or thumb is placed. These known apparatus have been disclosed in Japanese patent applications which are provisionally published under publication Nos. 55-13446 and 58-144280, respectively.
Before discussing in detail the known apparatus reference is firstly made to FIGS. 1 and 2. FIG. 1 shows light rays refracted and reflected at a boundary surface between two transparent substances I and II. In this instance I denotes air (refractive index Na) while II denotes an optical element which has a refractive index Np. In FIG. 1, a finger to be imaged is placed in contact with the surface or window 21 of the optical element 20. Assuming that .THETA.p and .THETA.a are respectively angles of incidence and refraction of a ray travelling along a path a-b-c, then the following equation is obtained using Snell's law. EQU Np sin .THETA.p=Na sin .THETA.a
It should be noted that in FIG. 1 each of .THETA.p and .THETA.a is less than a critical angle .THETA.c. Further, if a ray "d" is incident on the boundary surface at a point "b" at an angle .THETA.p' greater than .THETA.c, then it is totally internally reflected and travels in the optical element as shown by "e".
FIG. 2 illustrates a finger ridge 31 in contact with the surface 21 of the optical element 20. The refractive index of the surface of a finger is larger than that of air and is nearly equal to that of the optical element 20, so that a critical angle at the finger contact portion does not exist or approximates 90 degrees (denoted .THETA.c' in FIG. 2). It is therefore understood that a ray incident on a finger ridge from the optical element 20 does not totally internally reflect if the angle of incidence is smaller than .THETA.c'. In this case, part of the light ray incident on the finger ridge is absorbed in the finger, while the rest is reflected and scattered in various directions in the optical element 20.
With the above descriptions in mind, the two previously mentioned known apparatus will be discussed.
The first prior art arrangement is of non-scattering type and has been disclosed in the aforesaid Japanese patent application provisionally published under No. 55-13446. In this prior art, incident light rays are arranged to strike the surface of the optical element such that the light is totally internally reflected only at the portions where the finger ridges are not in direct contact with the window. A light receiving means (or a light detector) is placed in a manner to receive the totally internally reflected light, and hence a resultant fingerprint image wherein the ridges are dark and the background bright, is obtained.
On the other hand, the second prior art is of scattering type and has been disclosed in the above-mentioned Japanese patent application provisionally published under No. 58-144280. In this prior art, the incident rays strike the boundary surface such that any total reflection is not invited, and a light receiving means is positioned to receive only the rays scattered at the portions where finger ridges are in direct contact with the window. Therefore, the resulting fingerprint image is such that the finger ridge portions are bright on a dark background.
In the second prior art, however, if the incident rays are totally internally reflected only at the portions where finger ridges are not in contact, and, if the light receiving means is positioned to receive only the rays scattered at the finger ridges, the same fingerprint image can be obtained. That is to say, if the light source is positioned in a region defined by an angle &lt;gbg' in FIG. 2 by way of example, and if the light detector is positioned within the same angle region, the light detector receives only the scattered light rays.
However, both of these conventional fingerprint imaging apparatus have encountered the problem that if a finger to be examined is dry it is impossible to obtain a clear fingerprint impression. This will be discussed in detail with reference to FIGS. 3(A) and 3(B). As shown in these microscopical views, the surface of a finger ridge is not flat and mirror-like but uneven as illustrated. As a consequence, if a finger is wet with sweat (for example), the spaces between the surfaces of a finger and the refractive surface 21 are filled with an aqueous liquid. Since the index of sweat or water is equal to or almost equal to that of the optical element 20, a clear fingerprint image can be obtained. However, on the contrary, in the case that a finger is dry, a fingerprint impression with high contrast is no longer expected because there exist a number of air layers between the surfaces of a finger ridge and the window 20.